Thermal print heads, which have advantages such as low in noise, simple in maintenance or low running costs, have been widely used in various sorts of recording apparatuses including printers for use in facsimile apparatuses and word processors. In particular, thermal print heads providing high definition of more than about 400 dpi (dots per inch) have been used for stencil printing.
Among various thermal print heads, the ones which are for use in facsimile machines and word processor printers have been strongly demanded to have a finer heating resistor and an increased input energy density for the purpose of improving their resolution. Therefore, these thermal print heads are required to meet such a demand.
In order to satisfy the demand, the thermal print head is first required to have a heating resistor of a high resistive value.
As the materials of the heating resistor, cermet system resistors are widely used. Typical cermet systems are Ta--Si--O and Nb--Si--O. These materials are formed, for example, as a sputter film with use of a sputtering target prepared by mixing Ta and SiO.sub.2 powder and sintering thereof. At this time, a film having a resistivity of several m.OMEGA. to several tens m.OMEGA. can be formed while the amount of SiO.sub.2, sputtering pressure, etc. is controlled.
Meanwhile, for the purpose of obtaining a heating resistor having a high resistive value, there is a method of devising the design of the heating resistor. In the case of the thermal print head, however, it is desirable to make the sheet resistance of the sputter film high per se. To this end, it is considered to make the film thin to thereby have a high sheet resistance. However, making the film thickness small leads to a short life problem with the thermal print head. For the above reason, it is desirable that the film has a high resistivity.
Secondly, it is necessary for the resistive value of the heating resistor to fluctuate less when the heating resistor is used as a thermal print head or during the manufacture of the head on an assembly line.
A Ta--Si--O film has an excellent feature as a heating resistor, but tends to be influenced by its film forming conditions. Accordingly, when the film has a small resistivity, the film is made thinner, which leads to its poor life characteristics. When the film has a large resistivity, on the other hand, the film is required to be thicker, thus prolonging its film forming time. This also disadvantageously results in that the number of substrate films capable of being formed per target is decreased. From these reasons, a resistivity range is usually controlled to be usually about 10 to 20 m.OMEGA..multidot.cm for manufacturing of the film.
Even when the resistivity range of the heating resistor is limited as above, however, it has been found that, when the resistors are manufactured in the form of thermal print heads or devices, the devices have varying characteristics. This means that, even when the resistive films have an identical resistance, they may have respectively different structures. The film structure includes, for example, the degree or range of order and various other defect sorts and densities.
It is difficult to grasp such a film structure on a quantitative basis and to strictly control it. For example, with respect to a film deposited as it is and a film deposited and subjected to a vacuum process at a temperature of 900.degree. C., even when these films are subjected to a comparative analysis based on an X-ray diffractometry or a Raman spectroscopy, they fail to show a significant difference between them, exhibiting similarly broad amorphous patterns. For this reason, it has been difficult to realize a heating resistor having a good life characteristic and also to implement a thermal print head having a good life characteristic.
In addition to the above problem in the heating resistor, there is a further a problem that the resistive value of the thermal print head is non-uniform.
Making the heating resistor of the thermal print head finer and a correspondingly increased input energy density will entail a rise in the peak temperature of the central part of the heating resistor. Since a rise in the temperature generally causes a drop in the resistive value of the heating resistor, this requires a further increase in the input energy density, which further increases the temperature of the heating resistor, and further decreases the resistive value of the heating resistor. This process is repeated until the heating resistor eventually destroyed. Even when the increased temperature does not lead to its destruction, the decrease in the resistive value within the head or between the heads is not always uniform. A different decrease in the resistive value by the degree of the heating temperature results in irregularity in the printing density and quality determined.
The cause of the irregular drop in the resistive value is an insufficient thermal stability of the heating resistor and, in other words, the structural relaxation of the heating resistor is insufficient. As the thermal stabilizing measure, there is considered 1) a method for heating the heating resistor through its electrical conduction after assembling the heating resistor into a thermal print head, 2) a method for subjecting the heating resistor to a thermal process during or after the formation of the heating resistor, 3) a method for subjecting the heating resistor to irradiation of a high energy beam, 4) a method for subjecting the heating resistor to an induction heating process, or a similar method.
The measure 1) for thermally stabilizing the heating resistor is limited to its thermal stabilization level by IC rating and a reaction between the heating resistor and an electrode or protective film. For example, the thermal stabilization level is sufficient for the thermal print head for use in a facsimile equipment application but can be insufficient for use in plate making. The thermal stabilization measure 3) presents a problem from the viewpoint of its cost and productivity. The measure 4) is still in its experimental stage.
The thermal stabilization measure 2), which can thermally process the heating resistor without the problems associated with the IC and also the protective or electrode film, can set its thermal process temperature in a relatively wide range when compared to method 1), can be an excellent means from a comprehensive point of view, and can be partly put to practical use even in thermal print heads for use in plate making machines.
Conventionally, the thermal processing temperature has been mainly based on the temperature of the heating resistor at the time of driving the thermal print head as a rule of thumb. But in the case of a high-definition thermal print head for example, the temperature of the heating resistor was 800.degree. C. as its maximum, and the thermal process temperature was higher than the temperature of the heating resistor at the time of driving the thermal print head.
As, as already mentioned above, as the requirement of a finer heating resistor and larger input energy density increases, it is necessary to have a thermal print head to be driven at high temperature, and the thermal print head characteristics, process adaptability, etc. become different to a large extent depending on the type of glazing technology. Thus, even in the thermal stabilization measure 2) for performing thermal process at a temperature higher than the temperature of the heating resistor at the time of driving the thermal print head, there occurred problems which follow.
a) There are a wide range of variations in the resistive value of the heating resistor,
b) Upon manufacturing of the thermal print head, etching characteristics become deteriorated in a resistor film etching step.
c) The surface roughness of a glaze layer becomes bigger.
d) The anti-pulse life characteristic of the thermal print head deteriorates.
When these disadvantages become increased in number, it also becomes disadvantageously impossible to manufacture the thermal print head.
As also mentioned above, the finer patterning of the heating resistor of the thermal print head and the correspondingly increased input energy density entail the increase of the peak temperature of the central part of the heating resistor. As a result, when the structure of the heating resistor is fully relaxed, diffusing invasion of glaze layer components such as oxygen into the heating resistor increased. Thus, the heating resistor gradually increases in its resistive value and eventually becomes unusable. Further, when the heating resistor is driven under such conditions that the heating temperature becomes high, the resistive value of the heating resistor abruptly increases, whereby the thermal stress caused by a printing pulse may cause the heating part of the heating resistor to be released from the glaze layer. In this way, the rise of the heating temperature of the heating resistor causes not only the heating resistor to be chemically deteriorated but also mechanical destruction deteriorated, which can end in.
With respect to the aforementioned diffusing invasion of the glaze layer components into the heating resistor, such measures as will be mentioned below were considered.
(1) A barrier layer made of SiON or the like is provided between the glaze layer and heating resistor.
(2) There is employed a glazing material which is high in its thermochemistrical stability, i.e., high in its glass transition point.
(3) The layer of the heating resistor is made thick. That is, a relative length for the diffusing invasion length of glaze component grown during operation of the thermal print head is made short.
The above measure (1) presents a problem from the viewpoints of its productivity, cost and yield and thus impractical. The measure (2) is insufficient for the aforementioned demand because the glass transition point of 800.degree. C. becomes its technical upper limit from the viewpoint of maintaining the smoothness of the glaze. In the measure (3), when the layer thickness is simply increased, the resistive value drops. When, as the resistivity of the heating resistor layer is made high, it becomes difficult to obtain controllability over the resistive value and to manufacture a sputter target. When it is tried to cope with it by modifying the shape of the heating resistor layer, it becomes difficult to accurately perform patterning operation.
As mentioned above, any of the aforementioned measures has its own problems and cannot be a practical measure against the problem of the diffusing invasion of the glaze component into the heating resistor. Further, with regard to the problem of the heating part of the heating resistor being released from the glaze layer, no specific measure has been devised.